fix(diff): label split panes, unify the mode switch, sync split scrolling

Three things were wrong with the fresh side-by-side viewer:

- The panes scrolled independently. The sync hunted the editors' templated
  ScrollViewers at load time, but the viewer starts collapsed (no file selected),
  so nothing was ever measured, no template existed, and the hook silently
  no-opped for the lifetime of the window. Reading now goes through
  TextView.ScrollOffsetChanged, which exists from construction; writing goes
  through a lazily resolved ScrollViewer.Offset. TextEditor.ScrollToVerticalOffset
  is NOT usable here - it is a silent no-op in AvaloniaEdit 12.0.0 even with the
  editor templated (verified headlessly).
- Neither pane said what it showed. Added a BASE | WORKTREE header row inside
  DiffTextView, sharing the pane grid's columns so each label sits over its editor.
- The layout picker was two lookalike ToggleButtons. It is now a segmented switch
  (Border.segmented + Button.segment.active), with wrap demoted to an icon toggle
  since it is orthogonal to the layout mode.

The scroll sync, TextMate setup, grammar switching, brush fallback and segment
struct were duplicated between this control and the 3-pane conflict resolver,
which the design doc had used as a copy-paste template. They now live in
DiffEditorSetup; both surfaces migrated. They stay separate controls on purpose -
a read-only two-way renderer over aligned rows with filler lines is not a variant
of a three-way editor over a writable document.

The resolver thereby also picks up the fixed scroll sync and the TryFindResource
brush lookup (its TryGetResource never resolved anything). All four merge tokens
happen to equal their hardcoded fallbacks, so nothing changes on screen.

src/ClaudeDo.Ui/Design/IslandStyles.axaml

@@ -108,6 +108,9 @@
     <!-- Icon.Settings (gear) -->
     <StreamGeometry x:Key="Icon.Settings">M12 8a4 4 0 1 0 0 8 4 4 0 0 0 0-8z M19.43 12.98c.04-.32.07-.64.07-.98s-.03-.66-.07-.98l2.11-1.65a.5.5 0 0 0 .12-.64l-2-3.46a.5.5 0 0 0-.61-.22l-2.49 1a7.03 7.03 0 0 0-1.69-.98l-.38-2.65a.5.5 0 0 0-.5-.42h-4a.5.5 0 0 0-.5.42l-.38 2.65c-.61.25-1.17.59-1.69.98l-2.49-1a.5.5 0 0 0-.61.22l-2 3.46a.5.5 0 0 0 .12.64l2.11 1.65c-.04.32-.07.65-.07.98s.03.66.07.98l-2.11 1.65a.5.5 0 0 0-.12.64l2 3.46a.5.5 0 0 0 .61.22l2.49-1c.52.4 1.08.73 1.69.98l.38 2.65a.5.5 0 0 0 .5.42h4a.5.5 0 0 0 .5-.42l.38-2.65c.61-.25 1.17-.59 1.69-.98l2.49 1a.5.5 0 0 0 .61-.22l2-3.46a.5.5 0 0 0-.12-.64l-2.11-1.65z</StreamGeometry>
 
+    <!-- Icon.Wrap — a full-width line above a line that wraps back with an arrowhead -->
+    <StreamGeometry x:Key="Icon.Wrap">M3 5 H21 V7 H3 Z M3 11 H17 A4 4 0 0 1 17 19 H12 V21 L7 18 L12 15 V17 H17 A2 2 0 0 0 17 13 H3 Z</StreamGeometry>
+
     <!-- Icon.Skull — filled silhouette: rounded cranium + eye holes (EvenOdd) + jaw -->
     <StreamGeometry x:Key="Icon.Skull">F0 M12 2 C7 2 4 5.5 4 10 C4 13.5 6 16 8 17.5 L8 19 C8 20 8.9 21 10 21 L10 18.5 L14 18.5 L14 21 C15.1 21 16 20 16 19 L16 17.5 C18 16 20 13.5 20 10 C20 5.5 17 2 12 2 Z M8.5 8 L8.5 12 L11 12 L11 8 Z M13 8 L13 12 L15.5 12 L15.5 8 Z</StreamGeometry>
 
@@ -323,8 +326,9 @@
     <Setter Property="BorderBrush" Value="{StaticResource LineBrightBrush}" />
   </Style>
 
-  <!-- Icon button: 24×24 square with hover surface -->
-  <Style Selector="Button.icon-btn">
+  <!-- Icon button: 24×24 square with hover surface. The ToggleButton variant is the same
+       chrome for a persisted on/off control (its "on" state is the :checked rule below). -->
+  <Style Selector="Button.icon-btn, ToggleButton.icon-btn">
     <Setter Property="Width"           Value="24" />
     <Setter Property="Height"          Value="24" />
     <Setter Property="Padding"         Value="0" />
@@ -335,10 +339,53 @@
     <Setter Property="HorizontalContentAlignment" Value="Center" />
     <Setter Property="VerticalContentAlignment"   Value="Center" />
   </Style>
-  <Style Selector="Button.icon-btn:pointerover /template/ ContentPresenter">
+  <Style Selector="Button.icon-btn:pointerover /template/ ContentPresenter, ToggleButton.icon-btn:pointerover /template/ ContentPresenter">
     <Setter Property="Background" Value="{StaticResource Surface2Brush}" />
     <Setter Property="TextElement.Foreground" Value="{StaticResource TextBrush}" />
   </Style>
+  <Style Selector="ToggleButton.icon-btn:checked /template/ ContentPresenter">
+    <Setter Property="Background" Value="{StaticResource Surface3Brush}" />
+    <Setter Property="TextElement.Foreground" Value="{StaticResource AccentBrush}" />
+  </Style>
+  <!-- PathIcon fills with its own Foreground and does not pick up TextElement.Foreground,
+       so the icon's three states are set on the icon itself. -->
+  <Style Selector="ToggleButton.icon-btn PathIcon">
+    <Setter Property="Foreground" Value="{StaticResource TextMuteBrush}" />
+  </Style>
+  <Style Selector="ToggleButton.icon-btn:pointerover PathIcon">
+    <Setter Property="Foreground" Value="{StaticResource TextBrush}" />
+  </Style>
+  <Style Selector="ToggleButton.icon-btn:checked PathIcon">
+    <Setter Property="Foreground" Value="{StaticResource AccentBrush}" />
+  </Style>
+
+  <!-- Segmented switch: one control, mutually exclusive segments, active one highlighted.
+       Use for picking a mode (not for independent on/off flags — those are icon toggles). -->
+  <Style Selector="Border.segmented">
+    <Setter Property="Background"      Value="{StaticResource DeepBrush}" />
+    <Setter Property="BorderBrush"     Value="{StaticResource LineBrush}" />
+    <Setter Property="BorderThickness" Value="1" />
+    <Setter Property="CornerRadius"    Value="{StaticResource ButtonCornerRadius}" />
+    <Setter Property="Padding"         Value="2" />
+  </Style>
+  <Style Selector="Border.segmented Button.segment">
+    <Setter Property="Background"      Value="Transparent" />
+    <Setter Property="BorderThickness" Value="0" />
+    <Setter Property="Padding"         Value="10,4" />
+    <Setter Property="CornerRadius"    Value="4" />
+    <Setter Property="FontFamily"      Value="{StaticResource MonoFont}" />
+    <Setter Property="FontSize"        Value="{StaticResource FontSizeMono}" />
+    <Setter Property="Foreground"      Value="{StaticResource TextMuteBrush}" />
+  </Style>
+  <Style Selector="Border.segmented Button.segment:pointerover /template/ ContentPresenter">
+    <Setter Property="Background"             Value="{StaticResource Surface2Brush}" />
+    <Setter Property="TextElement.Foreground" Value="{StaticResource TextBrush}" />
+  </Style>
+  <!-- Declared after :pointerover so hovering the active segment keeps the active look. -->
+  <Style Selector="Border.segmented Button.segment.active /template/ ContentPresenter">
+    <Setter Property="Background"             Value="{StaticResource Surface3Brush}" />
+    <Setter Property="TextElement.Foreground" Value="{StaticResource AccentBrush}" />
+  </Style>
 
   <!-- Stroke-rendered icon (for line-art geometries that PathIcon would fill away) -->
   <Style Selector="Button.icon-btn Path.plan-icon">